A safety switching arrangement is used in switchgear in order to provide increased safety. The safety switching arrangement monitors an external electrical device and switches the power supply to a load according to the monitoring result. The monitored electrical device and the load can be the same item of equipment. The safety switching arrangement does not usually switch the load directly, but indirectly via special switching devices. In particular, the safety switching arrangement controls a load-specific switching device, e.g. a motor contactor or an auxiliary relay for such a motor contactor.
The safety requirements for a safety switching arrangement are set out in IEC 13849. This standard defines different categories, with Category 4 corresponding to the highest safety level. In order to achieve Category 4, it is necessary for all the safety-relevant components of the safety switching arrangement to be of redundant design.
In a safety-oriented safety switching arrangement, a safe control signal must be applied to the switch or switches. This means that the designers of a safety switching arrangement of this kind must have detailed knowledge of the operation of safety-oriented outputs of the safety switching arrangement. Depending on the intended purpose of the safety switching arrangement, a different design of those components of the safety switching arrangement which provide the safe control signal at the output may be necessary. In particular, for the control and evaluation of so-called “dark tests”, adaptation of the firmware implemented in the evaluation circuits is necessary. This procedure requires, on the one hand, valuable process time and, on the other, evaluation circuit resources.